The second ability can only exile a creature that's currently blocking or being blocked by Sisters of Stone Death. If Sisters of Stone Death isn't on the battlefield or have been removed from combat, the ability will be countered.

10/1/2005

The third ability can get back any creature card exiled by the second ability, no matter when it was exiled. If a card that's not a creature card (such as Svogthos, the Restless Tomb) was exiled, the third ability can't return it.